Have you heard this phrase and thought that it doesn’t sound quite right. It’s as if the truth of a rule is somehow strengthened by it not always being true. When you think more about it, this is nonsense. If there is a rule that says ‘all birds can fly’, the existence of a flightless bird like a penguin doesn’t prove the rule to be correct; in fact it proves just the opposite. So where does the phrase come from and what does it mean?
The phrase comes from a legal principle of republican Rome: exceptio probat regulam in casibus non exceptis (‘the exception confirms the rule in cases not excepted’). The first recorded citation is by the accomplished lawyer and orator, and later consul, Marcus Tullius Cicero, in his defence of Lucius Cornelius Balbus in 56BC who was charged with having been illegally granted Roman citizenship. The prosecution argued that as treaties with some non-Roman peoples explicitly prohibited them from becoming Roman citizens, it should be inferred in the treaty covering Balbus, even though it had no such clause. Cicero rejected this saying that if you prohibit something in certain cases, you imply that the rest of the time it’s permitted. The second part of Cicero’s phrase, in casibus non exceptis, or ‘in cases not excepted’, is almost always missing from modern uses of the statement that ‘the exception proves the rule’ and it is this that contributes to the confusion over the use of the phrase.
It is often suggested that it is the alternative meaning of the word ‘prove’ that is the source of the confusion, and that it means ‘to put to trial or to test’, as in ‘proving ground’ or ‘the proof of the pudding is in the eating’ etc. It could then be argued that the phrase means ‘it is the exception that tests whether the rule is true or not’. In the case of the ‘all birds can fly’ rule, it would mean that the existence of a bird that can’t fly would put the rule to the test, and find it wanting. Though the phrase can be used correctly in the context of scientific enquiry, it is not what the phrase meant originally.
Going back to the legal origin of the phrase, it’s the word ‘exception’ rather than ‘prove’ that causes the confusion. By exception we usually mean ‘something, not following a rule’ but in this case it means ‘the act of leaving out or ignoring’.
Cicero’s argument was that if an exception exists then there must be some rule to which the matter in question is an exception, that is the existence of an exception proves the existence of a general rule
If a sign says ‘entry is free of charge on Sundays’, you can reasonably assume that as a general rule, entry is charged for. So, from that statement, the rule would be that ‘you usually have to pay to get in’.
The exception on Sunday is demonstrating that the rule exists. It isn’t testing whether the rule ‘you have to pay’ is true or not, and it certainly isn’t proving the rule to be true.
Though ‘the exception proves the rule’ is a phrase best avoided, you might be thought pedantic to argue that it should only be used in its original Latin sense. Nowadays, when people use the phrase they may simply mean ‘every rule has an exception’ or ‘it’s an exception to the rule’. This may highlight the unusualness of an exception, a case that it is well outside the norm. The exception makes you notice what the norm is; its existence gives force to the rule. This is the gist of the original sense of the phrase.